1. Field of the Invention
The present invention relates to a radio-frequency (RF) accelerating cavity used in a charged-particle accelerator and a circular accelerator employing the RF accelerating cavity.
2. Description of the Background Art
Tuned RF accelerating cavities and untuned RF accelerating cavities are well-known examples of RF accelerating cavities used in a circular accelerator for accelerating electrically charged particles. To accelerate ions in an ion synchrotron using such an RF accelerating cavity, for example, an RF electric field whose frequency increases must be applied to the RF accelerating cavity fed from a RF power source to keep in step with the orbiting frequency of the ions.
A tuned RF accelerating cavity generates an accelerating voltage necessary for accelerating ions such that the resonant frequency of the cavity increases in synchronism with the frequency applied by the RF power source. In contrast, an untuned RF accelerating cavity is configured such that the impedance of the cavity is increased to necessary values in a full range of acceleration frequencies in advance.
There exist conventionally known techniques for controlling the resonant frequency of an RF accelerating cavity by the applied frequency in a predefined sequence. For example, Japanese Laid-open Patent Application No. 1995-006900 describes a tuned RF accelerating cavity which is structured such that ferrite members whose permeability has a large imaginary part are mounted in the RF accelerating cavity to decrease Q-value (or the ratio of a frequency range of resonance to a center frequency of resonance) of the RF accelerating cavity, and a bias coil for generating a magnetic field produced by the ferrite members is mounted in the cavity. In this structure, a real part of the permeability of the ferrite members is varied by the strength of a magnetic field produced by the bias coil to control the resonant frequency of an electromagnetic field excited in the RF accelerating cavity.
On the other hand, Japanese Laid-open Patent Application No. 1995-161500 describes an untuned RF accelerating cavity which is structured such that ferrite members having a large amount of Joule loss are mounted in the RF accelerating cavity to increase the impedance of the cavity by the ferrite members and a plurality of parallel shunt resistors are provided in the RF accelerating cavity. In this structure, the shunt resistors are switched in such a way that the shunt resistor having a large resistance value is connected to the ferrite members in a frequency range where the resistance value Zferr of the ferrite members is small and the shunt resistor having a small resistance value is connected to the ferrite members in a frequency range where the resistance value Zferr of the ferrite members is large, so that the RF accelerating cavity has a constant impedance throughout an acceleration frequency range thereof.
Also, Japanese Laid-open Patent Application No. 2001-126900 describes an untuned RF accelerating cavity which is structured such that an acceleration core made of ferrite is split into a plurality of segments cut by a plane containing a central axis of the acceleration core in order to decrease beam loading effects (or effects an ion beam exerts on the RF accelerating cavity) for uniformly accelerating the ion beam.
However, the aforementioned RF accelerating cavity of Japanese Laid-open Patent Application No. 1995-006900 has a problem that it is impossible to apply a high-intensity magnetic field as it is necessary to apply a direct current (DC) bias to the ferrite members and operate the RF accelerating cavity in the proximity of saturated magnetic field level of the ferrite members. Additionally, this RF accelerating cavity has a problem that the ferrite members can not be provided with any arrangement for cooling so that the inductance of the ferrite members is susceptible to temperature increase, making it difficult to control the RF accelerating cavity in a stable fashion.
In the aforementioned RF accelerating cavity of Japanese Laid-open Patent Application No. 1995-161500, it is necessary to set a resonance point in a range of acceleration frequencies, so that this RF accelerating cavity has a problem that it is impossible to set the impedance of an acceleration core at a desired point and sufficiently increase the impedance of the RF accelerating cavity.
Also, the aforementioned RF accelerating cavity of Japanese Laid-open Patent Application No. 2001-126900 has such problems as a cost increase due to the need for cutting the acceleration core into separate segments and heat generation caused by concentration of magnetic field at cut surfaces of the core segments particularly when the cavity is a large-core type.